HGF is a mesenchyme-derived pleiotrophic factor with mitogenic, motogenic and morphogenic activities on a number of different cell types. HGF effects are mediated through a specific tyrosine kinase, c-met, and aberrant HGF and c-met expression are frequently observed in a variety of tumors. See, e.g., Maulik et al., Cytokine & Growth Factor Reviews (2002), 13:41-59; Danilkovitch-Miagkova & Zbar, J. Clin. Invest. (2002), 109(7):863-867. Regulation of the HGF/c-Met signaling pathway is implicated in tumor progression and metastasis. See, e.g., Trusolino & Comoglio, Nature Rev. (2002), 2:289-300).
HGF binds the extracellular domain of the Met receptor tyrosine kinase (RTK) and regulates diverse biological processes such as cell scattering, proliferation, and survival. HGF-Met signaling is essential for normal embryonic development especially in migration of muscle progenitor cells and development of the liver and nervous system (Bladt et al., Nature (1995), 376, 768-771; Hamanoue et al., Faseb J (2000), 14, 399-406; Maina et al., Cell (1996), 87, 531-542; Schmidt et al., Nature (1995), 373, 699-702; Uehara et al., Nature (1995), 373, 702-705). Developmental phenotypes of Met and HGF knockout mice are very similar suggesting that HGF is the cognate ligand for the Met receptor (Schmidt et al., 1995, supra; Uehara et al., 1995, supra). HGF-Met also plays a role in liver regeneration, angiogenesis, and wound healing (Bussolino et al., J Cell Biol (1992), 119, 629-641; Matsumoto and Nakamura, Exs (1993), 65, 225-249; Nusrat et al., J Clin Invest (1994) 93, 2056-2065). The precursor Met receptor undergoes proteolytic cleavage into an extracellular α subunit and membrane spanning β subunit linked by disulfide bonds (Tempest et al., Br J Cancer (1988), 58, 3-7). The β subunit contains the cytoplasmic kinase domain and harbors a multi-substrate docking site at the C-terminus where adapter proteins bind and initiate signaling (Bardelli et al., Oncogene (1997), 15, 3103-3111; Nguyen et al., J Biol Chem (1997), 272, 20811-20819; Pelicci et al., Oncogene (1995), 10, 1631-1638; Ponzetto et al., Cell (1994), 77, 261-271; Weidner et al., Nature (1996), 384, 173-176). Upon HGF binding, activation of Met leads to tyrosine phosphorylation and downstream signaling through Gab1 and Grb2/Sos mediated PI3-kinase and Ras/MAPK activation respectively, which drives cell motility and proliferation (Furge et al., Oncogene (2000), 19, 5582-5589; Hartmann et al., J Biol Chem (1994), 269, 21936-21939; Ponzetto et al., J Biol Chem (1996), 271, 14119-14123; Royal and Park, J Biol Chem (1995), 270, 27780-27787).
Met was shown to be transforming in a carcinogen-treated osteosarcoma cell line (Cooper et al., Nature (1984), 311, 29-33; Park et al., Cell (1986), 45, 895-904). Met overexpression or gene-amplification has been observed in a variety of human cancers. For example, Met protein is overexpressed at least 5-fold in colorectal cancers and reported to be gene-amplified in liver metastasis (Di Renzo et al., Clin Cancer Res (1995), 1, 147-154; Liu et al., Oncogene (1992), 7, 181-185). Met protein is also reported to be overexpressed in oral squamous cell carcinoma, hepatocellular carcinoma, renal cell carcinoma, breast carcinoma, and lung carcinoma (Jin et al., Cancer (1997), 79, 749-760; Morello et al., J Cell Physiol (2001), 189, 285-290; Natali et al., Int J Cancer (1996), 69, 212-217; Olivero et al., Br J Cancer (1996), 74, 1862-1868; Suzuki et al., Br J Cancer (1996), 74, 1862-1868). In addition, overexpression of mRNA has been observed in hepatocellular carcinoma, gastric carcinoma, and colorectal carcinoma (Boix et al., Hepatology (1994), 19, 88-91; Kuniyasu et al., Int J Cancer (1993), 55, 72-75; Liu et al., Oncogene (1992), 7, 181-185).
A number of mutations in the kinase domain of Met have been found in renal papillary carcinoma which leads to constitutive receptor activation (Olivero et al., Int J Cancer (1999), 82, 640-643; Schmidt et al., Nat Genet (1997), 16, 68-73; Schmidt et al., Oncogene (1999), 18, 2343-2350). These activating mutations confer constitutive Met tyrosine phosphorylation and result in MAPK activation, focus formation, and tumorigenesis (Jeffers et al., Proc Natl Acad Sci USA (1997), 94, 11445-11450). In addition, these mutations enhance cell motility and invasion (Giordano et al., Faseb J (2000), 14, 399-406; Lorenzato et al., Cancer Res (2002), 62, 7025-7030). HGF-dependent Met activation in transformed cells mediates increased motility, scattering, and migration which eventually leads to invasive tumor growth and metastasis (Jeffers et al., Mol Cell Biol (1996), 16, 1115-1125; Meiners et al., Oncogene (1998), 16, 9-20).
Met has been shown to interact with other proteins that drive receptor activation, transformation, and invasion. In neoplastic cells, Met is reported to interact with α6β4 integrin, a receptor for extracellular matrix (ECM) components such as laminins, to promote HGF-dependent invasive growth (Trusolino et al., Cell (2001), 107, 643-654). In addition, the extracellular domain of Met has been shown to interact with a member of the semaphorin family, plexin B1, and to enhance invasive growth (Giordano et al., Nat Cell Biol (2002), 4, 720-724). Furthermore, CD44v6, which has been implicated in tumorigenesis and metastasis, is also reported to form a complex with Met and HGF and result in Met receptor activation (Orian-Rousseau et al., Genes Dev (2002), 16, 3074-3086).
Met is a member of the subfamily of receptor tyrosine kinases (RTKs) which include Ron and Sea (Maulik et al., Cytokine Growth Factor Rev (2002), 13, 41-59). Prediction of the extracellular domain structure of Met suggests shared homology with the semaphorins and plexins. The N-terminus of Met contains a Sema domain of approximately 500 amino acids that is conserved in all semaphorins and plexins. The semaphorins and plexins belong to a large family of secreted and membrane-bound proteins first described for their role in neural development (Van Vactor and Lorenz, Curr Bio (1999), 19, R201-204). However, more recently semaphorin overexpression has been correlated with tumor invasion and metastasis. A cysteine-rich PSI domain (also referred to as a Met Related Sequence domain) found in plexins, semaphorins, and integrins lies adjacent to the Sema domain followed by four IPT repeats that are immunoglobulin-like regions found in plexins and transcription factors. A recent study suggests that the Met Sema domain is sufficient for HGF and heparin binding (Gherardi et al., Proc Natl Acad Sci USA (2003), 100(21):12039-44).
As noted above, the Met receptor tyrosine kinase is activated by its cognate ligand HGF and receptor phosphorylation activates downstream pathways of MAPK, PI-3 kinase and PLC-γ(1, 2). Phosphorylation of Y1234/Y1235 within the kinase domain is critical for Met kinase activation while Y1349 and Y1356 in the multisubstrate docking site are important for binding of src homology-2 (SH2), phosphotyrosine binding (PTB), and Met binding domain (MBD) proteins (3-5), to mediate activation of downstream signaling pathways. An additional juxtamembrane phosphorylation site, Y1003, has been well characterized for its binding to the tyrosine kinase binding (TKB) domain of the Cbl E3-ligase (6, 7). Cbl binding is reported to drive endophilin-mediated receptor endocytosis, ubiquitination, and subsequent receptor degradation (8). This mechanism of receptor downregulation has been described previously in the EGFR family that also harbor a similar Cbl binding site (9-11).
Dysregulation of Met and HGF have been reported in a variety of tumors. Ligand-driven Met activation has been observed in several cancers. Elevated serum and intra-tumoral HGF is observed in lung, breast cancer, and multiple myeloma (12-15). Overexpression of Met and/or HGF, Met amplification or mutation has been reported in various cancers such as colorectal, lung, gastric, and kidney cancer and is thought to drive ligand-independent receptor activation (2, 16). Additionally, inducible overexpression of Met in a liver mouse model gives rise to hepatocellular carcinoma demonstrating that receptor overexpression drives ligand independent tumorigenesis (17). The most compelling evidence implicating Met in cancer is reported in familial and sporadic renal papillary carcinoma (RPC) patients. Mutations in the kinase domain of Met that lead to constitutive activation of the receptor were identified as germline and somatic mutations in RPC (18). Introduction of these mutations in transgenic mouse models leads to tumorigenesis and metastasis. (19).
Although the role of the Met kinase domain has been investigated in detail, and it has been theorized that increased expression levels of HGF/c-met probably underlie development of some cancers, direct evidence for a biological role for non-kinase domains of c-met has been lacking. Indeed, despite being implicated in the etiology of a variety of oncological conditions, the HGF/-c-met pathway has been a difficult pathway to target therapeutically. Efforts in this regard have been impeded in large part by a lack of understanding regarding mechanisms of action by which dysregulation of HGF/c-met causes tumorigenesis. Therefore, it is clear that the need for greater understanding of c-met-related oncogenic mechanisms of action is great. The invention provided herein meets this need and provides other benefits.
All references cited herein, including patent applications and publications, are incorporated by reference in their entirety.